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# -*- coding: utf-8 -*-
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# Natural Language Toolkit: CISTEM Stemmer for German
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# Copyright (C) 2001-2020 NLTK Project
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# Author: Leonie Weissweiler <l.weissweiler@outlook.de>
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# Algorithm: Leonie Weissweiler <l.weissweiler@outlook.de>
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# Alexander Fraser <fraser@cis.lmu.de>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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import re
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from nltk.stem.api import StemmerI
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class Cistem(StemmerI):
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"""
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CISTEM Stemmer for German
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This is the official Python implementation of the CISTEM stemmer.
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It is based on the paper
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Leonie Weissweiler, Alexander Fraser (2017). Developing a Stemmer for German
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Based on a Comparative Analysis of Publicly Available Stemmers.
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In Proceedings of the German Society for Computational Linguistics and Language
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Technology (GSCL)
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which can be read here:
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http://www.cis.lmu.de/~weissweiler/cistem/
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In the paper, we conducted an analysis of publicly available stemmers,
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developed two gold standards for German stemming and evaluated the stemmers
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based on the two gold standards. We then proposed the stemmer implemented here
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and show that it achieves slightly better f-measure than the other stemmers and
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is thrice as fast as the Snowball stemmer for German while being about as fast
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as most other stemmers.
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case_insensitive is a a boolean specifying if case-insensitive stemming
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should be used. Case insensitivity improves performance only if words in the
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text may be incorrectly upper case. For all-lowercase and correctly cased
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text, best performance is achieved by setting case_insensitive for false.
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:param case_insensitive: if True, the stemming is case insensitive. False by default.
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:type case_insensitive: bool
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"""
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strip_ge = re.compile(r"^ge(.{4,})")
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repl_xx = re.compile(r"(.)\1")
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strip_emr = re.compile(r"e[mr]$")
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strip_nd = re.compile(r"nd$")
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strip_t = re.compile(r"t$")
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strip_esn = re.compile(r"[esn]$")
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repl_xx_back = re.compile(r"(.)\*")
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def __init__(self, case_insensitive=False):
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self._case_insensitive = case_insensitive
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@staticmethod
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def replace_to(word):
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word = word.replace("sch", "$")
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word = word.replace("ei", "%")
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word = word.replace("ie", "&")
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word = Cistem.repl_xx.sub(r"\1*", word)
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return word
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@staticmethod
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def replace_back(word):
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word = Cistem.repl_xx_back.sub(r"\1\1", word)
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word = word.replace("%", "ei")
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word = word.replace("&", "ie")
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word = word.replace("$", "sch")
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return word
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def stem(self, word):
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"""
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This method takes the word to be stemmed and returns the stemmed word.
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:param word: the word that is to be stemmed
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:type word: unicode
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:return word: the stemmed word
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:rtype: unicode
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>>> from nltk.stem.cistem import Cistem
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>>> stemmer = Cistem()
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>>> s1 = "Speicherbehältern"
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>>> stemmer.stem(s1)
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'speicherbehalt'
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>>> s2 = "Grenzpostens"
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>>> stemmer.stem(s2)
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'grenzpost'
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>>> s3 = "Ausgefeiltere"
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>>> stemmer.stem(s3)
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'ausgefeilt'
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>>> stemmer = Cistem(True)
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>>> stemmer.stem(s1)
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'speicherbehal'
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>>> stemmer.stem(s2)
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'grenzpo'
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>>> stemmer.stem(s3)
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'ausgefeil'
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"""
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if len(word) == 0:
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return word
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upper = word[0].isupper()
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word = word.lower()
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word = word.replace("ü", "u")
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word = word.replace("ö", "o")
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word = word.replace("ä", "a")
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word = word.replace("ß", "ss")
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word = Cistem.strip_ge.sub(r"\1", word)
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word = Cistem.replace_to(word)
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while len(word) > 3:
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if len(word) > 5:
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(word, success) = Cistem.strip_emr.subn("", word)
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if success != 0:
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continue
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(word, success) = Cistem.strip_nd.subn("", word)
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if success != 0:
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continue
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if not upper or self._case_insensitive:
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(word, success) = Cistem.strip_t.subn("", word)
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if success != 0:
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continue
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(word, success) = Cistem.strip_esn.subn("", word)
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if success != 0:
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continue
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else:
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break
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word = Cistem.replace_back(word)
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return word
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def segment(self, word):
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"""
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This method works very similarly to stem (:func:'cistem.stem'). The difference is that in
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addition to returning the stem, it also returns the rest that was removed at
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the end. To be able to return the stem unchanged so the stem and the rest
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can be concatenated to form the original word, all subsitutions that altered
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the stem in any other way than by removing letters at the end were left out.
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:param word: the word that is to be stemmed
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:type word: unicode
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:return word: the stemmed word
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:rtype: unicode
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:return word: the removed suffix
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:rtype: unicode
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>>> from nltk.stem.cistem import Cistem
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>>> stemmer = Cistem()
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>>> s1 = "Speicherbehältern"
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>>> print("('" + stemmer.segment(s1)[0] + "', '" + stemmer.segment(s1)[1] + "')")
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('speicherbehält', 'ern')
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>>> s2 = "Grenzpostens"
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>>> stemmer.segment(s2)
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('grenzpost', 'ens')
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>>> s3 = "Ausgefeiltere"
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>>> stemmer.segment(s3)
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('ausgefeilt', 'ere')
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>>> stemmer = Cistem(True)
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>>> print("('" + stemmer.segment(s1)[0] + "', '" + stemmer.segment(s1)[1] + "')")
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('speicherbehäl', 'tern')
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>>> stemmer.segment(s2)
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('grenzpo', 'stens')
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>>> stemmer.segment(s3)
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('ausgefeil', 'tere')
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"""
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rest_length = 0
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if len(word) == 0:
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return ("", "")
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upper = word[0].isupper()
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word = word.lower()
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original = word[:]
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word = Cistem.replace_to(word)
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while len(word) > 3:
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if len(word) > 5:
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(word, success) = Cistem.strip_emr.subn("", word)
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if success != 0:
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rest_length += 2
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continue
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(word, success) = Cistem.strip_nd.subn("", word)
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if success != 0:
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rest_length += 2
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continue
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if not upper or self._case_insensitive:
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(word, success) = Cistem.strip_t.subn("", word)
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if success != 0:
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rest_length += 1
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continue
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(word, success) = Cistem.strip_esn.subn("", word)
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if success != 0:
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rest_length += 1
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continue
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else:
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break
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word = Cistem.replace_back(word)
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if rest_length:
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rest = original[-rest_length:]
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else:
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rest = ""
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return (word, rest)
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